The early detection and fine characterisation of defects, for example of cracks, in for example aeronautical structures, are a major issue for the safety and maintenance of aircraft.
Detection by means of eddy currents using electromagnetic sensors is made difficult by the complexity of the structure, which generates parasitic signals preventing the images from being easily read, by the low penetration of eddy currents due to the skin effect, by the low spatial resolution of the sensors which are generally used, and by the low number of measurement configuration options allowed by conventional sensors.
Conventionally, the detection of cracks in riveted joints is performed locally with point eddy current sensors or by means of constructions with 4 or 5 elements which operate differentially on a limited observation region of approximately 10 mm in diameter. With these devices, defects are sought at the base of the rivets, in the region of a second plate for example of a riveted assembly of aeroplane plates, at a depth of approximately 1.6 mm.
The small dimensions of the sensors or the small number thereof do not allow the structure to be explored in depth and also do not allow the position in the plates and the dimensions of the defect to be characterised precisely, as the spatial resolution of the signals to be detected is far too high.
This is why eddy current imagers which provide a larger observation region of approximately 76 mm and a higher spatial resolution have been developed.
These magneto-optic imagers are disclosed in particular in documents U.S. Pat. No. 5,053,704 and FR 2 856 791.
Meanwhile, the current image-treatment procedures, which are used by recently developed optical imagers, are ineffective for the fine detection of defects which are buried beyond a depth of 1.5 mm because of the binary nature of the images and/or the large number of perturbation patterns in the signal, which make interpretation of the images unreliable.
The objective problem is the poor performance provided by the presently used imaging procedures in detecting and finely locating defects which are buried deep inside a complex structure.